Filtration of sludges



July 11, 1961 H. SCHACKMANN ETAL 2,991,884

FILTRATIQN OF SLUDGES Filed Oct. 8, 1956 s Sheets-Sheet 1 FILTERING EFFICIENCY CURVE I. Co(OH) t=50 C II. Zn(OH) t=50C Precipitated from cobalt Precipitated from zinc chloride by calcium chloride solution by means hydroxide suspension, as in the end solution of zinc hydroxide suspension obtained from chlorinated roasted burnt pyrites Filter Capacity: kg/m /hour 200 400 600 800 I000 I200 I400 INVENTORS Heinrich Schackmann Carl Kayser BY Lgarst Albrecht 435 NEY July 11, 1961 H. SCHACKMANN ETAL 2,991,884

FILTRATION 0F SLUDGES Filed 001;. 8, 1956 8 Sheets-Sheet 2 Ca(OH) precipitated from calcium chloride solution by means of soda lye (NaOH) CaC| ZNOOH Ca (OH) ZNGCI FILTERI NG EFFICIENCY CURVE Ca (om --O- r 50 0 I 5 PRECIPITATED LIME I 2 N zoo 3 '5! '2 a 3 0 O E IO O 200 300 400 500 600 700 800 Filter Feed Concentration, moist solid substance 9/! FIG. 2.

INVENTORS Heinrich Schackmann Carl Kayser BY orst Albrecht dim ATTORNEY July 11, 1961 H. SCHACKMANN ETAL 2,991,884

FILTRATION OF SLUDGES Filed Oct. 8, 1956 v 8 Sheets-Sheet. 3

Test Filtration Curves for Examples of voluminous Compressible Sludges in the Class of Metal Hydroxide Precipitates in precipitation of the end solutions obtained from chlorinated roasted burnt pyrites Zn(OH) GOO Filter Press-Range Rotary Drum Filter-Range 3 l U 0 Co OH 3 500 )3 E 5 O .C m. 300

O O 8 0 Filter Press-Range ilter Press-Range Rotary Drum Filter-Range l l P3 l l l l 03' 0 I00 200 300 400 500 600 700 800 900 Filter Feed Concentration, dry solid substance g/l INVENTORS Heinrich Schockmann Carl Kayser 3. rst Albrecht By Ki a ATTORNEY July 11, 1961 Filed 001;. 8, 1956 kg Zn/m /h 8 Sheets-Sheet 4 lsoburs for I60 Torr filter vacuum 45o 1' I1 I: I" 35o y IOO Moist solid substance g/l INVENTORS c Heinrich Schockmunn Curl Koyser c 4 Horst Albrecht 40 c 0 y 11 TTORNE Y FILTRATION OF SLUDGES 8 Sheets-Sheet 5 Filed Oct. 8, 1956 lsobors for 260 Torr filrer vacuum Moist solid substance g/I INVENTORS 20C Heinrich Schuckmann Carl Koyser eoc Horst Albrecht Fig. 5.-

11 TTORNE Y y 1961 H. SCHACKMANN ETAL 2,991,834

FILTRATION 0F SLUDGES 8 Sheets-Sheet 6 Filed Oct. 8,- 1956 lsobors for 360 Torr filrer vacuum Moist solid substance g/l INVENTORS H. SCHACKMANN ETAL 2,991,884

July 11, 1961 FILTRATION OF SLUDGES 8 Sheets-Sheet 7 Filed Oct. 8, 1956 INVENTORS Heinrich Schackmunn Curl Koyser Fig. 7.

v; m 0 m w b 1 M v.

r o H y 1961 H. SCHACKMANN ETAL 2,991,884

FILTRATION OF SLUDGES Filed Oct. 8, 1956 8 Sheets-Sheet 8 Fi g 8 INVENTORS' Heinrich Schockmonn Curl Koyser Horsi Albrecht ATTORNEY United States Patent 2,991,884 FILTRATION OF SLUDGES Heinrich Schackmann, Carl Kayser, and Horst Albrecht, Duisburg, Germany Filed Oct. 8, 1956, Ser. No. 614,598 2 Claims. (Cl. 210-73) This invention relates to improvements in the filtration of sludges, and more particularly of those sludges which present particular filtration problems, especially sludges of metal hydroxide precipitates. Such sludges are known as voluminous and compressible, and in some respects they may also be said to be amorphous and colloidal, as well as non-crystalline, and perhaps slimy.

Typical examples of such voluminous and compressible sludges are Zn(OH) Ca(OH) Co(OH) and with respect to the specific filtration problems which they present, and for the purpose of this invention, they should be contrasted with sludges which are substantially crystalline and non-compressible, representative examples of which are sludges of coal, cement, and of ore, also sludges of magnesium carbonate, glass, ceramic substance, and bleaching earths.

It is Well known that each such voluminous compressible sludges are very diflicult to filter especially on the external type of continuous rotary filter, and that it is therefore generally necessary to filter them on filter presses necessitating relatively high pressures and tedious batch operation. (See Ost-Rassow, Lehrbuch der Chemischen Technology, 20th ed., p. 324.)

In the discussion of the rotary drum-filters that textbook states that these continuous rotary drurn filters are suited for handling fine-crystalline substances such as magnesium carbonate and the like, also (for calcium sludges and coal sludges if not too finely grained, also, for example, clays, ceramic substances, and for bleaching earths, but they are considered unsuited for handling precipitates having the voluminous, compressible, colloidal-amorphous, non-crystalline characteristics above indicated.

Therefore, a longstanding problemresides in the fact that such voluminous and compressible sludges of precipitates of metal hydroxides were substantially unfilterable upon other than the intermittent-1y operating filter presses. With respect to using the economically and operationally more desirable continuous external drum filters, only the addition of certain filter aids would render such sludges filterable, if at all.

Again, such filter aids would act as a contaminant where the filter cake itself represents the substance or value to be recovered in a state of attainable purity. Hence, the only other alternative, if any, was to operate the external drum-filter at extremely low capacity with the formation of a precariously thin filter cake. Therefore, because of the abovementioned characteristics of these sludges, workers in the art were confined to producing at best only a filter blanket upon the drum, so thin as to make continuous operation extremely difficult if not impossible, because of the resistance of such blanket to liquid permeation as well as because of the practical diificulties in continuously stripping or scraping oil such a thin cake encountered. In typical instances (as with Z11(OH) Ca(OH) and Co-(OH) a blanket thickness for instance in the order of was the best to be expected.

Consequently, it is among the objects of this invention to render sludges of that type filterable in a continuous economical manner upon external rotary drum filters, without necessitating the use of filter aids, avoiding the use of filter presses. That object has here been attained by the discovery that continuous filtration of such sludges 2 is practicable because of newly discovered favorable characteristic ratios of sludge concentratability versus filterability so that also an economically favorable ratio can be established as between the expenditures or efiorts for concentrating such sludges to a degree higher than heretofore thought economically feasible, and the gain in continuous filtration efliciency upon the external drum filter with continuous cake shipping.

This invention in practical filter operation is based upon the discovery that sludges, even those which seem to oifer the greatest inherent practical filtration problems, such as the metal hydroxide precipitates can in fact be filtered economically and successfully upon the external rotary drum filter as distinct from filter presses.

That discovery is best graphically expressed by a filtration efliciency curve with feed sludge concentration on the abscissa, and cake formation on the ordinate, that is grams of solids per liter of teed sludge on the abscissa and kg. of cake solids per square meter 0t filter surface in the ordinate. M

It has long been known in the filtration of the noncompressible crystalline sludges to apply the teed sludge to a continuous drum filter at a suitable high concentration to improve filtration efiiciency in proportion or even disproportionately.

Depending upon the characteristics of these normally filterable sludges such known filtration efliciency curves may represent a substantially linear function, or they may to some extent be convexly curved relative to the abscissa indicating a disproportionate increase in filtration efliciency with increase of teed sludge concentration. Hence, there has been little difiiculty in continuously filtering such sludges as contrasted with the voluminous compressible metal hydroxide precipitates.

The invention lies in the discovery that by concentrat ing these metal hydroxide precipitates sufliciently beyond the point or range normally considered practical and economical, that is beyond the conventional range of 300 x 400 g./l. moist substance, namely to a solids content of about 600 g./l.mois't' 'sub'st'anceand even toabout 800 g./l. or more, f.i. to 1400 -g./-l. the filterability or liquid permeability of the cake is changed unexpectedly and abruptly, whereby economical vacuum filtration on continuous external drum filters becomes practicable. In other words, from being normally unfilterable except on filter presses under high pressure, the characteristicsof these sludges change from. torming compressible filter cakes unsuited for drum filters to becoming continuously filterable on continuous external vacuum drum filters; In sludge of the Zn(OH) type, if concentrated or thickened to about 600 'g./l. moist weight or about 250 to 300 g./l. dry substance, will thereby attain what is here in termed the incipient critical filterability definable by a steeply ascending portion of the filtration efiiciency curve following a break. Thus, it should be understood that the elficiency curves herein dealt with are to be con sidered specifically in terms ot continuous filterability on a rotary drum filter. When so considered, it is a startling discovery that for instance the filterability characteristics of a Zn(OI-l) sludge are representable by a curve comprising a shallow ascending relatively linear initial portion and arelatively. sharp break followed by steeply ascending relatively linear outer end portion.

Considering the Zn(OH) filtration efficiency curve from.

that viewpoint, that curve then assumes the aspect of what should be termed a filterability? curve in -view.of-

V cally filterable along the steeply ascending portion following the break. Hence, in terms of rotary drum operation the initial portion preceding the break of that curve is in reality non-existent, while the existence of the break Withthe subsequent steeply ascending portion represents the discovery of a concentration range heretofore unknown in industrial filtration practice for such sludges, within which range continuous filtration becomes :feasible economically. Hence the discovery requires that the voluminous compressible sludges in question be subjected to a super concentration or super-thickening treatment exceeding that which was heretofore customary or considered economically attainable in connection with the filtration treatment of these sludges.

It is furthermore within the scope of this discovery that the break or incipient filterability increase is determinable for the type of voluminous compressible sludges here in question by means of a stationary test filter plate or vacuum filter cell placed in a suspension of the respective precipitate, with the filter cloth facing downwardly, while the cell is being moved uniformly in the suspension to stimulate continuous drum filter operation. By sequentially applying prepared suspensions of the material of increasing concentratiomthe filter eificiency can be plotted as a function of the concentration including the break or incipient filterability increase.

The data have thus been found to be directly analogous to the filtration behavior of the sludge in continuous rotary drum operation, so that this test curve thus becomes a filterability curve defining that concentration range at which continuous drum filtration becomes practicable and economically feasible in accordance with the discoveries of this invention. In this way by determining the break or incipient filterability increase for the suspended precipitate in question, there is ascertainable the continuous drum operating range that will produce cake thicknesses substantial enough to avoid the cake stripping and cloth clogging ditficulties above referred to.

An example of such a determination is as follows:

The test filter cell connected to a constant suction line is immersed in the suspension or bath of pulp while being moved therein in a uniform manner, while maintaining Such atest in fact produeesfiltration results close to those attainable bycontinuous filter drum operation, translatable into rotary .filter operation. Therefore, with 33% immersion of the filter drumand a suction period of 40 seconds, the rotary speed of the drum would be 30 revolutions per hour to produce comparable filtration results.

In other Words, a test filtration efficiency curve thus plotted for a representative voluminous compressible sludge such as Zn(OH) would reveal the sharp break beyond which that suspension must be concentrated or super-thickened inorder to avoid the use of batch operated filter presses, while rendering it economically suited for handling on the external rotary drum filter in continuous operation.

That is to say, such a filter cell test if interpreted in the sense of invention, reveals as in the case of Zn(OH) sludge, the initial shallow portion of the test curve prior to the break comprises the conventionally attainable and accepted concentration range requiring batch filter press operation, whereas the steep ascending portion of the curve past the break comprises the newlydiscovered concentration range which opens the door to economically practical continuous rotary drum filter operation for such sludges.

Such metal hydroxide precipitates as Zn(OH) and also Ca(OH) represent drastic examples of the discovery underlying this invention because of the pronounced sharpness of breaks the test filter curve as above 'defined, as well as because of the surprising degree of sudden steepness of the curve portion past the break. But of equal practical importance in that group of sludges is one which, such as Co(OH) has an initial portion of comparable shallow incline as well as a final portion of comparable steepness, the break occurring at a higher feed concentration than in the examples of Zn(OI-I) or Ca(OH) In the instance of such a voluminous compressible as is represented by a Co(OH) type of sludge, the test curve may be said to have a primary controlling significance with respect to conducting continuous filter operation.

For the purpose of this invention, one may consider such filtration efiiciency curves in terms of the continuous rotary drum filter operating ranges versus the conventional batch filter press operating ranges for the group or class of voluminous compressible sludges here under scrutiny, such contradistinction being presentable in the filtration curves herein presented.

For the respective sludges of voluminous comprwsible sludges a desired degree of concentration is attainable for instance by a mode of super thickening as disclosed in the co pending patent application Serial No. 400,832, filed December 29, 1953, and now abandoned, and briefly outlined as follows:

Such super thickening to the desired degree can be realized, for example, in a thickener tank provided with a rotary raking structure that has rake arms provided with sets of plowing raking blades, and a central sludge outlet means in the tank bottom. In this instance, the construction of the rake structure and the mounting of the blades upon the rake arms, as well as the depth of the tank, are such as to allow for the formatiton of a zone wise thickening pattern which comprises an upper clarification zone, a subjacent or intermediate sedimentation zone, and at the bottom a sludge zone or sludge bed. That is to say, raking blades are carried by the rake arms and are downwardly spaced therefrom by means of stems, so that there can be practiced a method of thickening whereby the sludge in the sludge zone or sludge bed is accumulated to a height significantly beyond that of the raking blades, to be detained in the sludge for a length of time beyond the start of the so-called compression phase.

The rotation of the structure causes the raking blades to convey the sludge towards the central discharge while being subjected to continuous kneading in a mechanical kneading zone directly at the tank bottom. In this way, the sludge in the kneading zone is overlain by a sludge bed zone that is a sludge detention or storage zone not reached by the blades, while the rake arms operate at an elevation substantially clear of the sludge bed accumulation.

In the practice of this thickening method, the volume of sludge being conveyed to the discharge point, is greater than the volume being discharged, so that the excess sludge thus conveyed is caused to move again outwardly in a portion of the sludge bed that lies above the kneading zone proper. Whatever the thus induced movement of the sludge may be within the sludge bed, the sludge therein is thus continuously turned over to aid in the release of liquid therefrom yet to prevent the remixing of the sludge in the kneading and conveying zone with liquid from the aforementioned intermediate sedimentatron zone.

Another mode of attaining greater than the conven- Particle Size, Weight, I: Percent The microscopic X-ray spectrographic examination of such sludges confirms the above size analyses, indicating that such sludges represent largely amorphous substances. An X-ray spectrogram of Co(OH) shows the same amorphous structure, and similar characteristics are observed with Ca(OH) At any rate, these analyses show that the physical characteristics of the sludge which are termed voluminous and are soft-compressible, differ radically from those which are readily filterable on drum filters, such as coal and ore.

lnsummary then, it is known that in contrast with non-compressible sludges, the voluminous compressible sludges may only be filtered with great difficulty. Filtering is generallyefiected with filter presses at increased pressure. It is generally not possible to use rotary external drum filters and the same when used are of only insuificient capacity. (See 0st Rassow, Lehrbuch der Chemischen Technologie, 20th Ed., page 324.)

One object of this invention is a new process for the filtration of voluminous compressible sludges which will allow the attaining of extremely high filtering efiiciencies by a combination of super-thickening and continuous filter operation. This and still further objects will become apparent from the following description.

It has been very surprisingly found that voluminous thin sludge of'the type herein considered each have a concentration of incipient critical filterability increase, i.e. a narrow limited concentration range which is well above 300-500 g./ l./ moist substance, which when reached will cause an exceedingly high increase of the filtering elficiency. This concentration of incipient efliciency curve by a point on the curve where the curve ascends steeply.

The concentration of incipient critical filterability increase according to this invention has been found to vary for each'type of sludge depending upon its chemicalv nature and the type of precipitate contained therein. The concentrationof incipient critical filterability increase is determined by plotting a filtering efliciency curve. This curve-has the rate of. filtration for a given filter area, as for example, in kilograms per square meter per hour plotted on the ordinate and the concentration of the sludge as for example, in grams per liter plotted on the abscissa. The concentration of incipient critical filterability increase is the concentration at which the curve begins to steeply ascend. Changes in temperature at which the sludges are filtered will only slightly change the position of the filtering efiiciency curve as a whole, but will not substantially alter the characteristics of the curve or of the point of concentration of incipient critical filterability increase.

'Exceedingly high filtering eificiencies are obtained with a given filter apparatus in accordance with the invention by concentrating the voluminous thin sludge to be filtered to at least the concentration of incipientcritical filterability increase and thereafter filtering the concentrated sludge.

As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the metes and bounds of the claims or of forms that are their functional as well as conjointly cooperative equivalents, are therefore intended to be embraced by those claims.

The drawings show by way of example filtering efiiciency curves for diflerent types of sludges.

FIGURE 1 shows curve I being the filtering eificiency curve of a cobalt hydroxide sludge precipitated by means of a Zinc-hydroxide suspension. Curve II is the filtering efliciency curve of a zinc hydroxide sludge precipitated by means of a calcium hydroxide suspension. curves are plotted for moist cake substance.

FIGURE 2 shows curve VI being the filtering efliciency.

curve of a lime sludge precipitated from 'a calcium chloride solution by means of soda lye, plotted for moist cake substance.

.FIGURE 3 shows filtration efiiciency curves for certain representative sludges plotted for dry cake substancesuper-thickening zone. 1 It will be noted that the FIGURE 1 and FIGURE 2 curves have the feed concentration plotted in the usual terms of moist substance from which the dry substance values can be derived at according to the usual moistures in Zn(OH) of 58% H O, in Ca(OH) of 60% H 0, and in Co(OH) of 50% H O. The feed concentration in the FIGURE 4 curves is plotted in terms of dry substance obtainable from the foregoing moisture values.

The above moisture values are largely independent of or unafiected by the degree of vacuum or temperature:

at which the suspension is being subjected to filtration.

As can be seen, each of these curves pass a point at which they begin to steeply ascend. This point read on the abscissa indicates the concentration of incipient critical filterability increase above which the sludge is concentrated in accordance with the invention.

When practicing the filtration process in accordance with this invention, and in conjunction with the specially, controlled super-thickening operation above outlined, it is possible to obtain filter cakes of highly concentrated pre-f cipitates which have a much lower moisture content than was conventionally obtainable.

of this invention contained only about 40% H O.

'Precipitates f.i. Fe(OH) containing about g./l.

of moist solid substance yield in accordance with conventionail processes afterfiltration, filter cakes with a moisture content of 50-60%.

30-35% are obtained.

The concentration or super-thickening of the solid sub- 2 These For example, for original cobalt hydroxide suspension containing about .30 g./l. moist solid substance, the filter cake obtainable in a conventional manner contains about 70% H O, whereas after super-thickening to about 1200-1400 g./l. "of moist substance the filter cake producible for the practice v After concentration to 1200 g./l. of moist solid substance in accordance with the in-' vention, filter cakes having a moisture content of only stance to the concentration of incipient critical filterability increase may be eifected with settling or thickening devices in the manner above set forth.

Considerable savings with respect to the filter area, filter cloth, and investments are effected when carrying out the process in accordance with the invention. The filter cakes obtained may be removed from the filter in a simple manner as, for example, by means of scrapers.

' The filtration efhciency curves of FIGURES 4, 5 and 6 all plotted for Zn(Ol-I) illustrate the fact that for the purpose of this invention, the moisture contents of the filter cake remains relatively constant, that is substantially independent of and unaffected by increases'in the feed concentration. With respect to Zn(OH) this is substantiated by the following tabulation.

Zinc Hydroxide Sludge, Temperature 50 Sludge Moist, Percent Hi 0.;Vacuum mm. Hg (=Torr.) g./l. of the Filter Cake FIGURE 4 shows a family of filtration efiiciency curves for Zn(OH) plotted for a filter vacuum of 160 Torr and temperatures of 20 C., 30 C., 40 C., 50 C., and 60 C. respectively.

FIGURE 5 shows a similar family of curves taken over a similar temperature range, although at a filter vacuum of 260 Torr.

FIGURE 6 shows a similar family of curves taken over a similar temperature, although at a filter vacuum of 360 Torr.

These graphs of FIGURES 4, 5 and 6 indicate that the filter capacity (kg/mfi/h.) is influenced to only a very small degree by the temperature of the suspension and by the degree of filter vacuum applied (m./m. Hg). The graphs have the solids concentrations in g./l. moist substance on the abscissa. Capacities are in kg. Zn(m. /h.) on the ordinate, with 100 kg. Zn being equivalent to about 400 kg. zinc sludge as moist substance.

Because of this independence of the filter capacity with respect to temperature within the practical range herein considered, it is observed in these graphs that for example, the curves plotted for 30 C. and for 50 C. respectively are located so close with respect to each other, that for practical purposes and within the margin of accuracy no significant change can be observed.

A method for super-thickening such as above referred to, for producing filter feed concentrations desired for the purposes of this invention, will now be outlined by comparison with conventional thickening and by reference to the schematic views of FIGURES 6 and 7.

In the operation of a conventional thickener there is a topmost zone of free settling in which all particles are free falling within the liquid and the settling velocity follows the law of Stokes. Below that zone there is a zone of hindered settling in which the solids and the liquid first move in opposite directions, then become relatively stationary while liquid still moves upwardly in between the particles, until finally both the solids and the liquid no longer move relative to one another. It is in this latter phase of the thickening pattern that conventional thickening operates and where re-mixing and re-suspension may occur of fine particles in the sludge that is normally conveyed by the rotary rakes in a very shallow bottom zone having dilute sludge directly above.

That is to say, in conventional thickening the raking blades operate in a layer of readily flowable sludges such that the mechanism in effect will serve merely to help the sludge move or slide down the inclined tank bottom towards the outlet, thus working one comparatively very thin layer of sludge when it has attained what prior prac tice considered final concentration. This is the aforementioned condition of concentration where the particles have attained that state of sedimentation in which they touch one another but will settle no further while having in fact attained that degree of concentration as well as flowability, at which they are invcntionally handled by the raking structure and discharged. This relatively thin bottom layer of sludge in conventional operation must be maintained as against a carefully controlled rate of sludge discharge, providing little if any sludge storage capacity. In other words, a relatively thin bottom layer of sludge still sufficiently flowable was accepted as the normal consistency (although dilute as compared with filter cake concentration), if the thickening operation was to proceed continuously and trouble-free.

Since in conventional thickener operation there is practically no storage sludge of discharge density, it necessitates careful control of the sludge discharge rate in accordance with the rate at which sludge is being conveyed by the rotary rakes to the outlet. However, in this phase of the conventional thickening and under the conditions outlined above, re-rnixing may occur of five sludge particles of the bottom layer with the more dilute sludge strata above, during conveyance of the sludge over the tank bottom.

By contrast and for the purposes of this invention, the suspension is super-thickened by the addition in the thickener of a super-thickening zone of appreciable and significant depth in addition and subjacent to the conventional sedimentation-thickening pattern above outlined. In this way, there is provided an additional zone for mixing, storage, and detention of the solids, also this provides an operational cushion or buffer, and furthermore an interposed protective zone against re-mixing of sludge particles from the bottom zone with more dilute strata above, as will be presently set forth.

In this super-thickening operation there is employed a rotary raking mechanism in which the raking blades proper are spaced downwardly from the rake arms by means of stems rigidly connecting the blades to the arms. In this way there is interposed between the rake arms and the raking blades proper what may be termed a detention-storing and mixing zone, or else a buffer and protection zone. In this added super-thickening zone the sludge at the bottom is conveyed by the raking blades towards the central storage outlet at a rate significantly greater than that of the sludge discharge with attendant turning over and mixing of the sludge within the superthickener zone. Referring to FIGURE 7, in the superthickening zone Z there are discernable a mechanical kneading zone Z at the very bottom and above it a mixing and storage zone Z In this way, the zone Z' is functionally interposed as a protective zone between the kneading zone 2;, below, and the conventional hindered settling thickening zone Z above. The kneading, with re-arranging of the particles, in zone Z' by the blades is effective under the added staticpressure of the sludge in Z.,, even while avoiding the re-mixing of super-thickened sludge of the bottom with thin sludges in the zones above.

,The disposition of the rotary rake structure is such that the rake arms extend a distance above the tank bottom sufiicient to be substantially above or clear of the super-thickening zone Z The following example is given to illustrate the plotting of a filter efliciency curve:

Example Five hundred cubic centimeters of a cobalt-hydroxide sludge obtained by precipitation of a cobalt chloride solution Co( OH) 3 with zinc hydroxide are filtered through a porcelain filter under vacuum until no more filtrate runs out'from the filter cake. The moist filter cake is weighed and the weight multiplied by two will give the number of? grams of solid substances per liter.

Samples of the substances having different predetermined solid concentrations in grams per liter were prepared from the starting sludge by thickening or concentrating it or by thinning it with appropriate quantities of its mother liquid. The concentration in grams per liter or solid substance was determined as set forth above.

The filtering efiiciency for the various samples so prepared having different pre-determined solid substance contents were determined, for example, by means of a hand filter plate of 150 square centimeters which is covered with a large mes'h fabric as a base and on top with a muslin cloth or any other filter cloth. The samples of the sludges of the different pre-determined solid content concentrations were sucked through the filters for a definite period of time and thereafter sucked dry. The filtering efiiciency per square meter per hour was calculated (according to the examples given above for a continuous- 1y working vacuum rotating filter whose filter drum dips in the sludge to be filtered) from the ascertained weight of the filter cake of the hand filtered press as follows:

Weight of the moist fi1ter 60 cake in kilog a s 150 Divided by the 3 filtration time in minutes The solid concentration in grams per liter of the moist sample was then plotted on the curse against the filtering efiiciency in kilograms per square meter per hour and the points of the various samples connected to obtain filtering efiiciency curve.

The filtration curves presented in FIGURE 4 are plotted for certain metal hydroxide precipitate sludges on the basis of grams per liter of dry solids, to indicate the contrast as between conventional feed concentration ranges requiring the use of batch filter presses, and the new feed concentration ranges enabling the same sludges to be handled on continuous external drum filters.

These curves are characterized substantially by an initial shallow ascending portion, a break, and steeply ascending portion. Thus, the curve VIII for Zn(OH) sludge has indicated thereon a tentative filter press operating range D the curve IX for Ca(OH) has indicated thereon a tentative filter press operating range P: as against a tentative drum filter operating range D the curve X for Co(OH) has indicated thereon a tentative filter press operating range P as against a tentative drum filter operating range D We claim:

1. The method of filtering suspensions of metal hydroxides of the group comprising Ca(OH) and Zn(OH) which comprises the steps of thickening said suspension to a sludge having a moist solids content in excess of a lower limit of about 600 grams per liter, where said suspensions while inoperable for continuous vacuum filtration at feed concentrations below said limit, are abruptly rendered continuously vacuum-filterable, and subjecting said thickened sludge to vacuum filtration on a rotary con tinuous vacuum type filter to produce a filter cake of substantial thickness adapted for continuous discharge from said rotary filter, which cake thickness increases from a point corresponding to said lower limit feed concentration along a steeply rising curve plotted with feed concentration indicated on the abscissa and with weight of filter cake per unit filter area per unit of time indicated on the ordinate.

2. The method of filtering a suspension of a metal hydroxide, namely Co(OH) which comprises the steps of thickening said suspension to a sludge having a moist solids content in excess of a lower limit of about 1000 grams per liter, where said suspensions while inoperable for continuous vacuum filtration at feed concentrations below said limit, are abruptly rendered continuously vacuum-filter-able, and subjecting said thickened sludge to vacuum filtration on a rotary continuous vacuum type filter to produce a filter cake of substantial thickness adapted for continuous discharge from said rotary filter, which cake thickness increases from a point corresponding to said lower limit feed concentration along a steeply rising curve plotted with feed concentration indicated on the abscissa and with weight of filter cake per unit filter area per unit of time indicated on the ordinate.

References Cited in the file of this patent UNITED STATES PATENTS 1,873,598 Jones Aug. 23, 1932 1,938,894 Darby et al Dec. 12, 1933 1,956,420 Gleason et al. Apr. 24, 1934 2,080,346 Tainton May 11, 1937 2,183,896 Rupp et al Dec. 19, 1939 2,185,785 Dorr et al. Ian. 2, 1940 2,441,584 McNeill May 18, 1948 2,481,110 Greenawalt Sept. 6, 1949 OTHER REFERENCES Kolthofi et al.: Textbook of Quantitative Inorganic Analysis, page 101 only, MacMillan Co., New York, 1947. 

1. THE METHOD OF FILTERING SUSPENSIONS OF METAL HYDROXIDES OF THE GROUP COMPRISING CA(OH)2 AND ZN(OH)2, WHICH COMPRISES THE STEPS OF THICKENING SAID SUSPENSION TO A SLUDGE HAVING A MOIST SOLIDS CONTENT IN EXCESS OF A LOWER LIMIT OF ABOUT 600 GRAMS PER LITER, WHERE SAID SUSPENSIONS WHILE INOPERABLE FOR CONTINUOUS VACUUM FILTRATION AT FEED CONCENTRATIONS BELOW SAID LIMIT, ARE ABRUPTLY RENDERED CONTINUOUSLY VACUUM-FILTERABLE, AND SUBJECTING SAID THICKENED SLUDGE TO VACUUM FILTRATION ON A ROTARY CONTINUOUS VACUUM TYPE FILTER TO PRODUCE A FILTER CAKE OF SUBSTANTIAL THICKNESS ADAPTED FOR CONTINUOUS DISCHARGE FROM SAID ROTARY FILTER, WHICH CAKE THICKNESS INCREASES FROM A POINT CORRESPONDING TO SAID LOWER LIMIT FEED CONCENTRATION ALONG A STEEPLY RISING CURVE PLOTTED WITH FEED CONCENTRATION INDICATED ON THE ABSCISSA AND WITH WEIGHT OF FILTER CAKE PER UNIT FILTER AREA PER UNIT OF TIME INDICATED ON THE ORDINATE. 